1. Field of the Invention
The present invention relates to a foil gas bearing which is used in a small gas turbine, a compressor, and the like. In particular, the present invention relates to a foil gas bearing which comprises top foils having a sufficient stiffness to reliably support a rotary shaft.
2. Description of the Related Art
It has been well known that a foil gas bearing is sometimes used as a bearing for supporting a rotary shaft of a gas turbine, a compressor, an expander, and the like. Foil gas bearings are classified into leaf-foil-type gas bearings and bump-foil-type gas bearings.
In the leaf-foil-type gas bearing, a plurality of top foils support the rotary bearing while they are supported by back springs. The top foil is provided in a rotary shaft retainer under conditions in which one end contacts the rotary shaft and the other end is fixed to the rotary shaft retainer, and the one end is more forward than the other end in a rotary direction of the rotary shaft. Specifically, the leaf-foil-type gas bearing will be explained referring to U.S. Pat. No. 4,195,395. FIG. 15 shows the conventional leaf-foil-type gas bearing described in U.S. Pat. No. 4,195,395. In FIG. 15, reference number 1 denotes a rotary shaft, and 2 denotes a leaf-foil-type gas bearing. The rotary shaft 1 is supported by the bearing 2. The bearing 2 comprises a tube shaped rotary shaft retainer 3 and a support ring 4 which is fixed to the inside of the rotary shaft retainer 3. Plural slots 5 are formed around the inside of the support ring 4. A plate shaped top foil 6 is fixed to the support ring 4 under the conditions in which one end is fixed to the slot 5 and the vicinity of the other end contacts the surface of the rotary shaft 1; that is, the top foil 6 is curved so as to partially contact around the rotary shaft 1, and the other end is more forward than the one end in a rotary direction of the rotary shaft 1. Each top foil 6 is supported by a support spring 7 which is fixed to the slot 5 from the outside of the top foil 6 in a radial direction of the rotary shaft 1.
The bump-foil-type gas bearing comprises a top foil and a bump foil in a corrugated plate shape. The bump-foil-type gas bearing supports the rotary shaft using the top foil and the bump foil. The bump-foil-type gas bearing will be explained referring to Japanese Patent Application, Examined Second Publication No. Hei 01-47649 (Japanese Patent Application, Unexamined First Publication No. Sho 59-093515). FIG. 16 shows the conventional bump-foil-type gas bearing disclosed in the Japanese Patent Application, Examined Second Publication No. Hei 01-47649. As shown in FIG. 16, a rotary shaft 11 is provided in a rotary shaft retainer 10. Around the rotary shaft 11, a ring shaped top foil 12 is provided. In addition, between the top foil 12 and the rotary shaft retainer 10, a bump foil in a corrugated plate shape 13 is provided. When the rotary shaft 11 rotates at high speed, air in a space between the rotary shaft 11 and the top foil 12 is involved in a rotation of the rotary shaft 11, and thereby, the rotary shaft 11 floats. In addition, the bump foil 13 has a resiliency, and therefore it exerts damping effects.
In the latter, when the rotary shaft 11 is displaced, the bump foil 13 rubs on the inside of the rotary shaft retainer 10, and thereby a frictional force is generated. Due to the frictional force, the damping effects can be obtained. In order to increase the frictional force, it is necessary to increase the rubbing distance over which the bump foil 13 rubs on the inside of the rotary shaft retainer 10. In addition, in order to increase the rubbing distance, it is necessary to substantially transform the bump foil 13. Due to this, a problem occurs in that the stiffness of the bump foil 13 decreases.
In the former, similar to the latter, in order to increase the damping effects by increasing the rubbing distance between the top foil 6 and the rotary shaft 1, it is necessary to increase the length of the top foil 6. In order to support the top foil 6 having a large length, a support spring 7 having a large length is needed. As a result, a problem occurs in that the stiffness of the support spring 7 decreases.
In addition, in conventional foil gas bearings, in order to decrease the frictional force which is caused by contacting the rotary shaft 1 or 11 and the top foil 6 or 12 when the operation starts or stops, the surface of the rotary shaft 1 or 11 is chrome plated and a calcination film containing fluororesin as a main component is formed on the surface of top foil 6 or 12. However, since the maximum allowable temperature of the calcination film containing fluororesin is low, such as about 200xc2x0 C., it is difficult to use foil gas bearings comprising the top foils which are coated with the calcination film with elements for gas turbines which are subjected to high temperatures.
The present invention has been realized in consideration of the matters mentioned above, and an object of the present invention is to provide a foil gas bearing which has sufficient damping effects due to a frictional force and a sufficient stiffness to support the rotary shaft.
In addition, another object of the present invention is to provide a foil gas bearing in which a back spring for preventing the displacement of the rotary shaft has a sufficient stiffness to prevent damages to the back spring; and in which a frictional force which is generated by the contact between the rotary shaft and the top foil when the operation starts or stops, decreases.
In addition, another object of the present invention is to provide a foil gas bearing in which the back spring for preventing the displacement of the rotary shaft is easily mounted into the rotary shaft retainer with a high precision, and thereby a bearing clearance between the rotary shaft and the top foil is maintained precisely, and in which the bearing clearance can be adjusted after assembly.
Furthermore, another object of the present invention is to provide a foil gas bearing which has a high maximum allowable temperature and an improved heat resistance, and which can be used for the elements of gas turbines which are subjected to high temperatures.
In order to achieve the above objects, the present invention provides a foil gas bearing comprising a rotary shaft retainer for retaining a rotary shaft, and a back spring for preventing the displacement of the rotary shaft, which is provided between the rotary shaft and the rotary shaft retainer. Plural protrusions are formed at one element which is either the rotary shaft retainer or the back spring. Plural recesses are formed at the other element, which is either the back spring or the rotary shaft retainer, at places which correspond to the protrusions, and the protrusions partially fit into the recesses.
According to the foil gas bearing, when the rotary shaft is displaced in a radial direction or in a thrust direction thereof, the protrusion which is formed at the back spring (or the rotary shaft retainer) fits the recess which is formed at the rotary shaft retainer (or the back spring). Due to this, a frictional force is generated, and the frictional force has damping effects on the displacement of the rotary shaft. In addition, when a recess and a protrusion fit, a tensile force or a compressive stress is generated at the back spring. Specifically, when the back spring comprises the protrusions, a tensile force is generated. In contrast, when the back spring comprises the recess, a compressive stress is generated. Thereby, the stiffness of the back spring is improved and it is possible to oppose the displacement of the rotary shaft.
In the foil gas bearing, it is preferable to further have a top foil for preventing the displacement of the rotary shaft in the radial direction thereof, which is provided between the rotary shaft and the back spring.
According to the foil gas bearing, when the rotary shaft is displaced in the radial direction thereof and a biasing force is applied to the back spring via the top foil, the protrusion and the recess are fitted together, which are formed at the back spring and the rotary shaft retainer. A frictional force is thereby generated between the recess and the protrusion and damping effects occur for the displacement of the rotary shaft in the radial direction thereof. In addition, when the recess and the protrusion are fitted together, a tensile stress or a compressive stress is generated, and this reliably improves the stiffness of the back spring. Therefore, it is possible to oppose the displacement of the rotary shaft in the radial direction thereof.
In the foil gas bearing, it is preferable for the recess to have a wedge shape and to be formed around the inner surface of the rotary shaft retainer, for the back spring to have a ring shape, and for the protrusion to be formed at the ring shaped back spring so as to protrude outwardly.
According to the foil gas bearing, when the rotary shaft is displaced in the radial direction thereof and a biasing force is applied to the back spring via the top foil, the protrusion which is formed at the back spring fits the recess which is formed at the rotary shaft retainer. Due to this, a frictional force is generated and damping effects occur for the displacement of the rotary shaft. In addition, when the recess and the protrusion are fitted together, a tensile force is generated at the back spring between the protrusions.
In the foil gas bearing, it is preferable for the recess to be formed obliquely such that the bottom of the wedge shaped recess is oriented more forward in the rotation direction of the rotary shaft with respect to a radial line of the rotary shaft on which the center of the recess lies, as viewed from the rotary shaft.
According to the foil gas bearing, since the recess is formed obliquely such that the bottom of the wedge shaped recess is oriented more forward in the rotation direction of the rotary shaft with respect to the radial line of the rotary shaft on which the center of the recess lies, as viewed from the inside of the rotary shaft retainer, due to the turning force of the rotary shaft, the protrusion of the back spring reliably enters the recess of the rotary shaft retainer. Therefore, damping effects for the displacement of the rotary shaft in the radial direction can be reliably obtained.
In the foil gas bearing, it is preferable for the recess to be formed at the back spring so as to protrude toward the center of the rotary shaft, and for the protrusion to be formed at the inner surface of the rotary shaft retainer. In the foil gas bearing, it is more preferable for the recess to have a wedge shape.
According to the foil gas bearing, when the rotary shaft is displaced in the radial direction thereof and a biasing force is applied to the back spring via the top foil, the back spring is pressed toward the outside in the radial direction of the rotary shaft. The recess of the back spring catches the protrusion of the rotary shaft retainer. Then, a frictional force is generated between the recess and the protrusion and damping effects occur for the displacement of the rotary shaft. In addition, when the recess and the protrusion fit with each other, a compressive stress is reliably generated between the protrusions of the back spring.
In the foil gas bearing, it is preferable that plural support members be formed at the outside of the top foil, and that recesses be formed at the support member so as to protrude toward the center of the rotary shaft, that plural protrusions be formed at the back spring so as to protrude toward the center of the rotary shaft at place which corresponds to the recess, and that the protrusion partially fits into the recess.
According to the foil gas bearing, when the rotary shaft is displaced in the radial direction and a biasing force is applied to the back spring via the top foil, the top foil is pressed toward the outside in the radial direction of the rotary shaft. Then, the protrusion formed at the back spring is pressed into the recess formed at the support member. A frictional force is thereby generated between the recess and the protrusion and damping effects occur for the displacement of the rotary shaft in the radial direction thereof. In addition, when the recess and the protrusion fit together, a compressive stress is reliably generated between the protrusions of the back spring.
In the foil gas bearing, it is preferable for the top foil to comprise plural leaf foils of which one end, that is, a base portion, is fixed to the back spring between the protrusions, and for the rotary shaft to be supported by the other end, that is, a tip portion, of the leaf foils.
According to the foil gas bearing, when the rotary shaft is displaced in the radial direction and a biasing force is applied to the back spring via the leaf foils, the protrusion formed at the back spring is pressed into the recess formed at the rotary shaft retainer. Then, when the protrusion is gradually pushed into the recess, a frictional force is generated and damping effects occur for the displacement of the rotary shaft in the radial direction thereof. In addition, when the recess and the protrusion fit with each other, a tensile force is reliably generated between the protrusions of the back spring.
In the foil gas bearing, it is preferable for the back spring to comprise plural recesses which protrude toward the rotary shaft retainer and to be provided in the rotary shaft retainer so as to maintain a gap between the outer surface of the back spring and the inner surface of the rotary shaft retainer between the recesses. The top foil preferably comprises plural top foil elements which are divided in the circumferential direction of the rotary shaft. The top foil element to comprises protrusions which protrude toward the recess of the back spring at one end thereof, and the recess of the back spring and the protrusion of the top foil element to partially fit with each other.
According to the foil gas bearing, when the rotary shaft is displaced in the radial direction and a biasing force is applied to the back spring via the top foil elements, the protrusion formed at the top foil element is pressed into the recess formed at the back spring. Then, when the protrusion is gradually pushed into the recess, a frictional force is generated and damping effects occur for the displacement of the rotary shaft in the radial direction thereof In addition, when the recess and the protrusion fit with each other, a tensile force is reliably generated between the recesses of the back spring. The stiffness of the back spring is thereby improved and it is possible to oppose the displacement of the rotary shaft.
In the foil gas bearing, it is preferable for the back spring to comprise notches at both ends in a longitudinal direction thereof, which notches extend toward the center of the axial line of the rotary shaft.
According to the foil gas bearing, the radial stiffness, that is, the stiffness, for supporting the rotary shaft, of the back spring at both ends in a longitudinal direction thereof can be decreased. If the rotary shaft inclines and a biasing force is applied to the end of the back spring, since the radial stiffness at the end of the back spring is relatively small, the back spring can deform flexibly in response to the biasing stress due to the rotary shaft. The back spring and rotary shaft thereby contact at a large contact area. Therefore, it is possible to avoid applying a large stress at a local portion of the rotary shaft.
In the foil gas bearing, it is preferable for the rotary shaft to comprise a small diameter portion and a large diameter portion with an end face between the small diameter portion and the large diameter portion. The rotary shaft retainer is preferably provided at the small diameter portion; and the back spring is preferably provided between the end face of the rotary shaft and the rotary shaft retainer and has a plate shape.
According to the foil gas bearing, when the rotary shaft is displaced in the thrust direction thereof, the protrusion and the recess, which are formed at the rotary shaft retainer and the back spring, fit with each other. A frictional force is thereby generated and damping effects occur for the displacement of the rotary shaft in the thrust direction thereof In addition, when the recess and the protrusion fit with each other, a tensile force or a compressive stress is generated in the back spring. The stiffness of the back spring is thereby improved and it is possible to oppose the displacement of the rotary shaft.
In addition, in order to achieve the above objects, the present invention provides another foil gas bearing comprising a rotary shaft retainer for retaining a rotary shaft, and a back spring for preventing the displacement of the rotary shaft, provided between the rotary shaft and the rotary shaft retainer. The rotary shaft retainer comprises plural recesses of which the cross section is quadrangular. The back spring comprises plural recesses such that it enters inside of the recesses of the rotary shaft retainer, connection portions between the recesses, and peripheral inclined portions each of which connects the recess and the connection portion which separate them as they extend toward the, rotary shaft so as to form a gap between the outer surface of the connection portion and the inner surface of the rotary shaft retainer. The back spring comprises notches at both ends in a longitudinal direction thereof and at the recess and the peripheral inclined portions and a part of the connection portion, which extend toward the center of the back spring in the longitudinal direction.
In the foil gas bearing, it is preferable to have plural top foils for supporting the rotary shaft provided between the rotary shaft and the back spring. The top foil to comprises a wedge portion comprising an edge and two inclined portions extending from the edge toward the rotary shaft at one end and an extending portion which extends from the wedge portion so as to surround the rotary shaft. It is provided inside of the back spring so that the edge of the wedge portion is inside of the recess formed at the back spring and inclined portions of the wedge portion contact the peripheral inclined portions of the back spring.
In addition, in order to achieve the above objects, the present invention provides another foil gas bearing comprising a rotary shaft retainer for retaining a rotary shaft, and a back spring for preventing the displacement of the rotary shaft, provided between the rotary shaft and the rotary shaft retainer. The rotary shaft retainer comprises plural recesses of which the cross section is quadrangular. The back spring comprises plural recesses which enter inside of the recesses of the rotary shaft retainer, connection portions between the recesses, and peripheral inclined portions each which connects the recess and the connection portion which separate them as they extend toward said rotary shaft so as to form a gap between the outer surface of the back spring and the inner surface of the rotary shaft retainer. The back spring comprises plural notches at the connection portion, which extend toward the center of the back spring in the longitudinal direction.
In the foil gas bearing, it is also preferable to further have plural top foils for supporting the rotary shaft, which are provided between the rotary shaft and the back spring. The top foil to comprises a wedge portion comprising an edge and two inclined portions extending from the edge toward the rotary shaft at one end and an extending portion which extends from the wedge portion so as to surround the rotary shaft. It is provided inside of the back spring so that the edge of the wedge portion is inside of the recess formed at the back spring and the inclined portions of the wedge portion contact the peripheral inclined portions of the back spring.
According to this foil gas bearing, the radial stiffness, that is, the stiffness for supporting the rotary shaft of the back spring at both ends in a longitudinal direction thereof, can be decreased. If the rotary shaft inclines and a biasing stress is applied to the end of the back spring, since the radial stiffness at the end of the back spring is relatively small, the back spring can flexibly deform in response to the biasing stress on to the rotary shaft. The back spring and rotary shaft thereby contact at a large contact area. Therefore, it is possible to avoid applying a large stress at the local portion of the rotary shaft.
In addition, in the foil gas bearing, it is preferable to coat a solid lubricant at the contact surfaces between the back spring and the rotary shaft retainer, and the rotary shaft retainer and the top foil. In addition, it is also preferable to coat the solid lubricant on the rotary shaft.
According to the foil gas bearing, it is possible to control the lubricating conditions between these members and to protect the sliding portions of these members.
In addition, in order to achieve the above objects, the present invention provides another foil gas bearing comprising a rotary shaft retainer for retaining a rotary shaft, a top foil for preventing the displacement of the rotary shaft, which is provided around the rotary shaft, and a back spring for preventing the displacement of the rotary shaft, provided between the top foil and the rotary shaft retainer. The top foil has a thin plate shape and comprises plural top foil elements which are divided in the circumferential direction of the rotary shaft. One end of the top foil element is fixed to the back spring. The back spring has a ring shape formed by fixing both ends of a thin plate at a connection portion under a conditions in which the back spring has a tensile force, wherein one end has a protrusion portion and the other end has a recess portion which catches the protrusion portion, and the width of the protrusion portion substantially equals the width of the recess portion.
According to the foil gas bearing, since the protrusion portion formed at the one end is caught by the recess formed at the other end and the width of the protrusion portion substantially equals the width of the recess portion, and the side surfaces (restriction surfaces) of the protrusion portion contact the inner side surfaces (restriction surfaces) of the recess portion, the protrusion portion cannot move in the width direction of the back spring in the recess portion. In addition, since the protrusion portion of the back spring crosses over the recess portion at the connection portion, the circumferential distance of the back spring can be easily adjusted by varying the cross length between the protrusion portion and the recess portion. In other words, the circumferential extent of the back spring can be easily adjusted by varying the cross point between the protrusion portion and the recess portion. Therefore, it is possible for the back spring to be provided stably in the rotary shaft retainer without a part of the back spring contacting the inside of the rotary shaft retainer. In addition, when the circumferential distance of the back spring varies, since one end of the top foil element is fixed to the back spring, the position of the top foil element changes in the radial direction of the rotary shaft. Therefore, it is possible to maintain a gap between the rotary shaft and the top foil at an appropriate distance by adjusting the circumferential distance of the back spring.
In the foil gas bearing, it is preferable for the rotary shaft retainer to comprise a recess and inclined portions which extend toward the rotary shaft from both ends of the recess in the circumferential direction. The back spring is preferably provided in the rotary shaft retainer so that the vicinity of the connection portion contacts the inclined portions of the rotary shaft retainer and the cross point between the protrusion portion and the recess portion is in the recess formed in the rotary shaft retainer.
According to the foil gas bearing, since the vicinity of the connection portion of the back spring contacts the inclined portions of the rotary shaft retainer, the back spring can be positioned exactly with respect to the rotary shaft retainer. In addition, since the cross point between the protrusion portion and the recess portion is in the recess formed in the rotary shaft retainer, there is a space sufficient to adjust the position of the cross point and the cross point can be adjusted easily with high precision, and the gap between the rotary shaft and the top foil can also be adjusted easily with high precision.
In the foil gas bearing, it is preferable to further have a back spring diameter adjusting mechanism comprising an adjusting groove which is provided at the inner surface of the rotary shaft retainer and a pressing piece which is provided in the adjusting groove and moves outwardly and inwardly in the radial direction of the rotary shaft.
In the foil gas bearing, it is preferable for the rotary shaft retainer to comprise plural recesses and inclined portions which extend toward said rotary shaft from both ends of said recess in the circumferential direction at the inner surface, for the back spring to comprise plural protrusion portions which protrude toward said rotary shaft retainer, and for the back spring to be provided in the rotary shaft retainer so that the protrusion portions contacts the inclined portions of the rotary shaft retainer.
In the foil gas bearing, it is preferable to further comprise plural back spring diameter adjusting mechanisms, for the pressing piece of the back spring diameter adjusting mechanism to comprise a recess which opens toward said rotary shaft and a tapered surfaces which are formed at the both sides of the recess in the circumferential direction of the rotary shaft, for the back spring to have plural protrusion portions and to be provided said rotary shaft retainer such that the edges of said protrusion portions and said connection portion is in the recess of said pressing piece and said protrusion portions and said connection portion contact said tapered portions of said pressing piece.
According to the foil gas bearing, the diameter of the back spring can be adjusted by moving the pressing piece in the radial direction of the rotary shaft. Specifically, when the pressing piece moves inwardly, the diameter of the back spring increases. In contrast, when the pressing piece moves outwardly, the diameter of the back spring decreases. Therefore, the gap between the rotary shaft and the top foil can be adjusted even after the rotary shaft, the top foil, the back spring, and the like are provided in the rotary shaft retainer, and thereby the foil gas bearing is assembled. In addition, the gap between the rotary shaft and the top foil can be adjusted with even higher precision by adjusting the position of the pressing piece. Furthermore, the assembling steps for the foil gas bearing and the adjusting steps for the gap between the rotary shaft and the top foil can decrease.
In the foil gas bearing, it is preferable that plural recesses be formed at the surface of the top foil, which faces to the rotary shaft or the surface of the rotary shaft, or at both the surface of the top foil, which faces to the rotary shaft and the surface of the rotary shaft.
According to the foil gas bearing, since a solid lubricant can be provided in the recesses, the surface of the rotary shaft can be lubricated.
In the foil gas bearing, it is preferable for the top foil to be coated with a film containing at least one selected from the group consisting of graphite and molybdenum disulfide (MoS2).
In addition, in the foil gas bearing, it is preferable for the top foil to be coated with a film containing at least two metallic oxides.
Furthermore, in the foil gas bearing, it is also preferable for the top foil to be coated with a film containing graphite and at least two metallic oxides.
According to these foil gas bearings, it is possible to maintain the coefficient of friction between the rotary shaft and the top foil low in a wide temperature range from the ordinary temperature to high temperatures, such as about 600xc2x0 C. While the rotary shaft contacts the top foil, such as during starting, during low revolutions, and the like, it is possible to avoid abrasion loss of the rotary shaft and the top foil from increasing over such a wide temperature range. In addition, since the maximum allowable temperature of the top foil increases, it is possible to use the foil gas bearings comprising the top foil for elements for of gas turbines which are subjected to high temperatures.
In addition, in order to achieve the above objects, the present invention provides a foil gas bearing structure comprising a rotary shaft, and a rotary shaft retainer for retaining a rotary shaft, a top foil for preventing the displacement of the rotary shaft; which is provided around the rotary shaft, and a back spring for preventing the displacement of the rotary shaft, which is provided between the top foil and the rotary shaft retainer. The rotary shaft is coated with a hard carbon, and the top foil is coated with a calcination film containing fluororesin.
According to the foil gas bearing structure, since a hard carbon film has solid lubricating properties which are greater than those of a chromium plating film and a ceramic film, a hard carbon film is suitable for coating the surface of the rotary shaft. When the surface of the rotary shaft is coated with hard carbon and the top foil is coated with a calcination film containing fluororesin, transitional lubricating effects can be obtained between the surface of the rotary shaft and the surface of the top foil. In other words, the lubricant at the rotary shaft easily transfers to the surface of the top foil. Therefore, the abrasion resistance of the foil gas bearing is improved, and the service life of the foil gas bearing increases.
In addition, in order to achieve the above objects, the present invention provides another foil gas bearing comprising a rotary shaft retainer for retaining a rotary shaft, a top foil for preventing the displacement of the rotary shaft; which is provided around the rotary shaft, and a back spring for preventing the displacement of the rotary shaft, which is provided between the top foil and the rotary shaft retainer. The top foil is coated with a film containing at least one selected from the group consisting of graphite and molybdenum disulfide (MoS2).
In addition, in order to achieve the above objects, the present invention provides another foil gas bearing comprising a rotary shaft retainer for retaining a rotary shaft, a top foil for preventing the displacement of the rotary shaft, which is provided around the rotary shaft, and a back spring for preventing the displacement of the rotary shaft, which is provided between the top foil and the rotary shaft retainer. The top foil is coated with a film containing at least two metallic oxides.
In addition, in order to achieve the above objects, the present invention provides another foil gas bearing comprising a rotary shaft retainer for retaining a rotary shaft, a top foil for preventing the displacement of the rotary shaft, which is provided around the rotary shaft, and a back spring for preventing the displacement of the rotary shaft, which is provided between the top foil and the rotary shaft retainer. The top foil is coated with a film containing graphite and at least two metallic oxides.
According to these foil gas bearings, it is possible to maintain the coefficient of friction between the rotary shaft and the top foil low over a wide temperature range from ordinary temperatures to high temperatures, such as about 600xc2x0 C. While the rotary shaft contacts the top foil, such as during starting, during low revolutions, and the like, it is possible to avoid abrasion loss of the rotary shaft and the top foil from increasing over such a wide temperature range. In addition, since the maximum allowable temperature of the top foil is increased, it is possible to use the foil gas bearings comprising the top foil for elements of gas turbines which are subjected to high temperatures.